The present invention relates to a process for the continuous, solvent- and mastication-free production of pressure-sensitive self-adhesive compositions based on non-thermoplastic elastomers, using tackifier resins, typical rubber plasticizers, optionally fillers and heat-activatable crosslinkers, and to the coating thereof to produce self-adhesive articles, in particular high-performance self-adhesive tapes.
Fundamental to the profile of performance requirements of pressure-sensitive adhesive systems and the pressure-sensitive adhesive articles produced with them are the two physical phenomena of adhesion and cohesion. Adhesion is dealt with in the technical jargon using the terms instant bond strength (tack) and bond strength (peel strength) and describes by definition the terms xe2x80x9cself-adhesivexe2x80x9d and/or xe2x80x9cpressure-sensitive adhesivexe2x80x9d, i.e. permanent adhesive bonding under xe2x80x9cgentle pressurexe2x80x9d.
Especially in the case of pressure-sensitive adhesives based on natural rubber, this property is obtained by mixing in tackifier resins (tackifiers) and plasticizers having relatively low molecular weights.
The second defining property of the pressure-sensitive adhesives is their simple residue-free redetachability after use. This property is determined essentially by the high molecular mass rubber fractions as the elastomer component, which give the system, in the form of cohesion (internal strength), the required strength under shear stress, which is of particular significance for the use of the products at relatively high temperatures and/or mechanical loads. By additional crosslinking, for example by way of ionizing rays, reactive resin components or other chemical crosslinkers, this property can be reinforced.
The performance of the pressure-sensitive adhesive is, therefore, critically determined by the balanced proportion of adhesion properties and cohesion properties and by compatibility, homogeneity and stability of the blend of components with extremely high and relatively low average molecular weights, something which is relatively easy to achieve in the case of production of the composition in industry-standard mixers and kneading machines using solvents.
The solvent-free compounding and processing of self-adhesive compositions, on the other hand, has become established primarily only for the processing of elastomers which melt, so-called thermoplastic elastomers.
In this case, the process of producing the composition is normally conducted in the melt in twin-screw extruders at relatively high temperatures, with coating taking place normally by means of slot dies.
The advantage of using thermoplastic elastomers lies essentially in a simplification of the coating process. The avoidance of flammable solvents does away with the need for the drier units, with their high energy consumption for the evaporation and recovery of the solvents, and to use explosion-protected units. Hot-melt coating units are compact and permit much higher coating speeds. Moreover, the technology is an environment-friendly one in which there are no solvent emissions.
For the solvent-free compounding of thermoplastic elastomers, the prior art makes use predominantly of block copolymers having polystyrene block fractions. The advantage of this class of substance is that the polystyrene domains present in the polymer soften above 100xc2x0 C., accompanied by a sharp fall in the viscosity of the adhesive composition and thereby providing ease of processing. After cooling to room temperature, the polystyrene domains are reformed and impart a certain shear strength to the pressure-sensitive adhesives based on thermoplastic elastomers.
The thermoplastic elastomers can be compounded faultlessly in the extruder process using hydrocarbon resins which promote bond strength. In this way, a desired level of bond strength can be achieved with relative ease. The resultant pressure-sensitive adhesives, however, remain sensitive to temperatures above 40xc2x0 C. For the self-adhesive tapes produced on this basis, this remanent xe2x80x9ccreep behaviourxe2x80x9d is critical for unrestricted storage stability (blocking of the rolls in the stack, especially in the course of transportation in relatively warm climate zones) and for their use at relatively high operating temperatures (for example as masking tapes in automotive finishing, where despite postcrosslinking such tapes lose their functional capacity: the pressure-sensitive adhesive softens and the shear strength for fixing the masking papers is no longer ensured).
For this reason, the known hot-melt pressure-sensitive adhesives based on block copolymers have been able to establish themselves almost exclusively for packaging tapes and for labels for use at room temperatures.
Using non-thermoplastic elastomers, such as natural rubber, on the other hand, it is possible to achieve the required shear strengths; however, the solvent-free production and processing of natural-rubber pressure-sensitive adhesives has to date confronted the person skilled in the art with unsolved problems.
Owing to the extremely high molecular mass fractions of the rubber (with Mwxe2x89xa71 million), solvent-free self-adhesive compositions cannot be processed by the hot-melt pressure-sensitive adhesive technology, or else the rubbers used must be reduced in their molecular weight (broken down) so greatly before processing that as a result of this breakdown their suitability for high-performance self-adhesive compositions is impaired.
The deliberate industrial process of rubber breakdown under the combined action of shear stress, temperature and atmospheric oxygen is referred to in the technical literature as mastication and is generally carried out in the presence of chemical auxiliaries, which are known from the technical literature as masticating agents or peptizers, or, more rarely, as xe2x80x9cchemical plasticizing aidsxe2x80x9d. In rubber technology, the mastication step is necessary in order to make it easier to integrate the additives.
This mastication must be clearly distinguished from the breakdown known as degradation which results in all of the standard solvent-free polymer technologies, such as compounding, conveying and coating in the melt. Uncontrolled degradation often constitutes an unwanted phenomenon. It can be minimized by providing an inert gas atmosphere.
A variety of routes to the solvent-free production and processing of rubber pressure-sensitive adhesives have been described.
The patent CA 698 518 describes a process for achieving production of a composition by adding high proportions of plasticizer and/or simultaneously strong mastication of the rubber. Although this process can be used to obtain pressure-sensitive adhesives having an extremely high tack, user-compatible shear strength, even with a relatively high level of subsequent crosslinking, can be achieved only to a limited extent owing to the relatively high plasticizer content or else to the severe breakdown in molecular structure of the elastomer to a molecular weight average of Mwxe2x89xa61 million.
The use of polymer blends, where besides non-thermoplastic natural rubber use is also made of block copolymers, in a ratio of approximately 1:1, is essentially an unsatisfactory compromise solution, since it results neither in high shear strengths when the self-adhesive tapes are used at relatively high temperatures nor in significant improvements relative to the properties described in the patent.
JP 07 324 182 A2 describes a multistage process in which a double-sided adhesive tape has a pressure-sensitive adhesive layer based on an acrylic resin adhesive and has a second layer comprising a blend of isoprene-styrene elastomer, natural rubber and non-reactive hydrocarbon resin (Arkon P 100). This tape is used as a carpet-laying tape, where there are likewise no stringent requirements on the shear strength at elevated temperatures.
The use of non-thermoplastic elastomers is also described in JP 95 331 197, where use is made of an isocyanate-reactive natural rubber (polyisoprene grafted with maleic ester) having an average molecular weight of below 1 million with aliphatic non-reactive hydrocarbon resins, which is crosslinked with blocked isocyanates (for example Desmodur CT); the mixture is initially crosslinked at 150xc2x0 C. for five minutes and following its subsequent coating onto PET film is cured at 180xc2x0 C. for several minutes (for example 15 minutes). This procedure clearly shows how complicated it is to achieve postcrosslinking if the natural rubber is subjected to excessive breakdown during the production process.
The patent application JP 95 278 509 protects a self-adhesive tape in which the natural rubber is masticated to an average molecular weight of Mw=100,000 to 500,000 in order to obtain a coatable homogeneous mixture comprising hydrocarbon resins, rosin/rosin-derivative resins and terpene resins, which can be processed readily at between 140xc2x0 C. and 200xc2x0 C. with a coating viscosity of from 10 to 50xc3x97103 cps but require an extremely high subsequent EBC dose (40 Mrad) in order to ensure the shear strength necessary for their use. For carrier materials such as impregnated and/or sized papers, and for woven carriers based on viscose staple and the like, the system is not very suitable, since at the necessarily high beam doses there is significant carrier deterioration.
The use of exclusively non-thermoplastic rubbers as the elastomer component in the formulation of pressure-sensitive adhesives with the existing cost advantage possessed by, for example, natural rubbers over the standard commercial block copolymers, and the outstanding properties, especially the shear strength of natural rubber and of corresponding synthetic rubbers, is also set out at length in the patents WO 94 11 175, WO 95 25 774, WO 97 07 963 and, correspondingly, U.S. Pat. Nos. 5,539,033 and 5,550,175. In these cases, the additives customary in pressure-sensitive adhesive technology, such as tackifier resins, plasticizers and fillers, are described.
The production process disclosed in each case is based on a twin-screw extruder which permits compounding to a homogeneous pressure-sensitive adhesive blend with the chosen process regime, involving mastication of the rubber and subsequent gradual addition of the individual additives with an appropriate temperature regime.
The mastication step of the rubber, which precedes the actual production process, is described at length. It is necessary and characteristic of the process chosen, since with the technology chosen therein it is indispensable to the subsequent integration of the other components and to the extrudability of the blended composition. Also described is the feeding in of atmospheric oxygen, as recommended by R. Brzoskowski, J. L. and B. Kalvani in Kunststoffe 80 (8), (1990), p. 922 ff., in order to accelerate mastication of the rubber.
This procedure makes it absolutely necessary to practise the subsequent step of electron beam crosslinking (EBC) and to use reactive substances as EBC promoters in order to achieve an effective crosslinking yield.
Both process steps are described in the abovementioned patents, but the EBC promoters chosen also tend towards unwanted chemical crosslinking reactions at elevated temperatures, which limits the use of certain tackifier resins.
Owing to the unavoidable high product temperatures, compounding in a twin-screw extruder prevents the use of heat-activatable substances suitable for crosslinking the adhesive compositions, such as, for example, reactive (optionally halogenated) phenolic resins, sulphur or sulphur-donor crosslinker systems, since the chemical crosslinking reactions which ensue in the process result in such a great increase in viscosity that the coatability of the resulting pressure-sensitive adhesive composition is impaired.
In summary, it can be stated that all of the known processes are characterized by extremely severe rubber breakdown. When the compositions are processed further into self-adhesive tapes, this necessitates extreme crosslinking conditions and results, moreover, in a partially restricted applications profile, especially as regards the use of resultant self-adhesive tapes at elevated temperatures.
There is numerous known apparatus for the continuous production and processing of solvent-free polymer systems. In common use are screw machines such as single-screw and twin-screw extruders in a variety of process lengths and with a variety of fittings. However, continuously operating kneading apparatus of a very wide variety of constructions, including, for example, combinations of kneading and screw machines, or else planetary roll extruders, are also used for this purpose.
Planetary roll extruders have been known for a fairly long time and were first used in the processing of thermoplastics such as PVC, for example, where they were used primarily to supply the downstream units such as, for example, calenders or roll mills. As a consequence of their advantage of the great renewal of surface area for material exchange and heat exchange, by means of which the frictional energy can be dissipated rapidly and effectively, and because of the low residence time and the narrow residence-time spectrum, their use in recent times has been extended, inter alia, to compounding processes which require a particular temperature-controlled regime.
Depending on manufacturer, planetary roll extruders are available in various designs and sizes. The diameters of the roll cylinders, depending on the desired throughput, are typically between 70 mm and 400 mm.
Planetary roll extruders generally have a filling section and a compounding section. The filling section consists of a conveying screw to which all of the solid components are fed continuously. The conveying screw then passes the material to the compounding section. The area of the filling section, together with the screw, is preferably cooled in order to prevent baking-on of materials on the screw. Alternatively, there are designs without a screw section, where the material is fed directly between central spindles and planetary spindles. However, this is not important for the effectiveness of the process of the invention.
The compounding section consists of a driven central spindle and a number of planetary spindles which rotate around the central spindle within a roll cylinder with internal helical gearing. The rotary speed of the central spindle and hence the rotational speed of the planetary spindles can be varied and is therefore an importance parameter for the control of the compounding process. The materials are circulated between the central and planetary spindles, or between the planetary spindles and the helical gearing of the roll section, so that under the influence of shear energy and external heating the materials are dispersed to form a homogeneous compound.
The number of planetary spindles rotating in each roll cylinder can be varied and thus adapted to the requirements of the process. The number of spindles influences the free volume within the planetary roll extruder, and the residence time of the material in the process, and also determines the surface area for heat and material exchange. By way of the shear energy introduced, the number of planetary spindles has an influence on the result of compounding. Given a constant diameter of roll cylinder, a larger number of spindles permits better homogenization and dispersion or, respectively, a greater product throughput.
The maximum number of planetary spindles installable between the central spindle and the roll cylinder depends on the diameter of the roll cylinder and on the diameter of the planetary spindles used. When using relatively large roll diameters, as required for obtaining production-scale throughputs, and/or relatively small diameters for the planetary spindles, the roll cylinders can be equipped with a relatively large number of planetary spindles. With a roll diameter of D=70 mm, typically up to seven planetary spindles are used, whereas with a roll diameter of D=200 mm ten, for example, and with a roll diameter of D=400 mm 24 for example, planetary spindles can be used.
In this context, reference is made to the patent applications and, respectively, utility model DE 196 31 182, DE 94 21 955, DE 195 34 813, DE 195 18 255 and DE 44 33 487, which give an overview of the prior art in the field of planetary roll extruders.
Thus, furthermore, DE 39 08 415 A1 describes the processing of rubber mixtures or rubberlike mixtures by means of planetary roll extruders. For the purpose of further processing on downstream equipment, pre-batches or finished mixtures are masticated and plastified on a planetary roll extruder. Likewise described is the production of finished mixtures in a planetary roll extruder: in this case vulcanizing systems and other components are metered into the rubber premixes.
The object of the present invention was to provide a process with which pressure-sensitive self-adhesive compositions based on non-thermoplastic elastomers can be produced continuously without solvent, with or without the use of thermally reactive components, and, if desired, can be coated in-line without the need for property-impairing mastication of the rubber.
The invention accordingly provides a process for the continuous solvent-free and mastication-free production of self-adhesive compositions based on non-thermoplastic elastomers in a continuously operating apparatus having a filling section and a compounding section, comprising the following steps:
a) feeding the solid components of the self-adhesive composition, such as elastomers and resins, into the filling section of the apparatus, optionally feeding of fillers, colorants and/or crosslinkers,
b) transferring the solid components of the self-adhesive composition from the filling section to the compounding section,
c) adding the liquid components of the self-adhesive composition, such as plasticizers, crosslinkers and/or further tackifier resins, optionally in the melted state, to the compounding section,
d) preparing a homogeneous self-adhesive composition in the compounding section, and
e) discharging the self-adhesive composition.
It has been found particularly advantageous to use, as the continuously operating apparatus, a planetary roll extruder whose compounding section consists preferably of at least two, but with particular preference three, coupled roll cylinders, it being possible for each roll cylinder to have one or more separate temperature control circuits.